In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
LTE is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation (4G) of mobile telecommunication networks. In comparisons with third generation (3G) WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE technology is a mobile broadband wireless communication technology in which transmissions are sent using orthogonal frequency division multiplexing (OFDM), wherein the transmissions are sent from base stations, also referred to herein as network nodes or eNBs, to mobile stations, also referred to herein as user equipments or UEs. The transmission OFDM splits the signal into multiple parallel sub-carriers in frequency.
A basic unit of transmission in LTE is a Resource Block (RB) which in its most common configuration comprises 12 subcarriers and 7 OFDM symbols in one time slot. A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE), as shown in FIG. 1. Thus, an RB comprises 84 REs.
Accordingly, a basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies for subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized sub-frames, #0-#9, each with a Tsub-frame=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
An LTE radio sub-frame is composed of multiple RBs in frequency with the number of RBs determining the bandwidth of the system and two slots in time, as shown in FIG. 3. Furthermore, the two RBs in a sub-frame that are adjacent in time are denoted as an RB pair.
Downlink (DL) transmissions are dynamically scheduled, i.e. in each subframe the network node transmits control information about to which user equipments data is transmitted and upon which RBs the data is transmitted, in the current DL subframe. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. The number n=1, 2, 3 or 4 is known as a Control Format Indicator, CFI. The DL subframe also comprises common reference symbols, CRS. The CRS are known to the receiver and used for coherent demodulation of e.g. the control information.
Carrier Aggregation
In LTE Release 10, a Component Carrier (CC) bandwidth of up to 20 MHz is supported. This is the maximal carrier bandwidth for the earlier LTE Release 8. Hence, an LTE Release 10 operation that is wider than 20 MHz is possible. To a UE of LTE Release 10, this may appear as a number of LTE carriers.
However, it may also be advantageous to assure that an efficient use of a wide carrier is also performed for legacy UEs, i.e. where legacy UEs may be scheduled in all parts of the wideband LTE Release 10 carrier. One way to do is by means of Carrier Aggregation (CA), as shown in FIG. 4.
In the LTE Release 10, up to 5 aggregated carriers is supported. Each carrier is limited in the Radio Frequency (RF) specifications to have one out of six bandwidths, namely, 6, 15, 25, 50, 75 or 100 RBs. These correspond to 1.4, 3, 5, 10, 15 and 20 MHz, respectively.
The number of aggregated CCs, as well as, the bandwidth of the individual CC may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL are the same. An asymmetric configuration refers to the case where the number of CCs in DL and UL are different.
Note that the number of CCs configured in the network node may be different from the number of CCs as seen by a UE. For example, a UE may support more DL CCs than UL CCs, even though the network node offers the same number of UL CCs and DL CCs.
CCs may also be referred to as cells or serving cells. Particularly, in an LTE network, the CCs aggregated by a UE may be denoted Primary Cell (PCell) and Secondary Cells (SCells). The term “serving cell” may comprise both a PCell and SCells. The PCell is UE specific and may be viewed as “more important”. That is because vital control signaling and other important signaling is typically handled via the PCell. The CC configured as the PCell is the primary CC, whereas all other CCs are secondary CCs.
During initial access a UE of LTE Release 10 acts similarly to a UE of LTE Release 8. For example, upon successful connection to the network a UE may, depending on its own capabilities and the networks capabilities, be configured with additional CCs in the UL and DL. This configuration may be based on Radio Resource Control (RRC) signalling.
Because of heavy signalling and rather slow speed of the RRC signalling, a UE may be configured with multiple CCs, even though not all of the CCs are currently being used. If a UE is activated on multiple CCs, it follows that the UE has to monitor all DL CCs, e.g. for a Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDCCH). This implies a wider receiver bandwidth, higher sampling rates, etc. This will result in higher power consumption by the UE.
Timing Alignment of Signals Received at the Network Node
In order to preserve the orthogonality in UL, the UL transmissions from multiple UEs need to be time aligned at the network node. This means that the transmit timing of the UEs, which are under the control of the same network node, should be adjusted to ensure that their transmitted signals arrives at the network node at the same time. More specifically, well within the Cyclic Prefix (CP). CP may e.g. be seen as a guard interval for the symbols in the signals. This ensures that the network node is able to use the same resources, i.e. the same DFT or FFT resource, to receive and process the signals from multiple UEs.
As shown in FIG. 5, UEs may be located at different distances from the network node. In FIG. 5, the UE 522 is located closer to the network node 510 than the UE 521, Because the UE 521 and the UE 522 are located at different distances from the network node 510, the UE 521 and the UE 522 will need to initiate their UL transmissions at different times. A UE far from the network node 510, i.e. UE 521, needs to start transmission earlier than a UE close to the network node 510, i.e. UE 522. This may, for example, be handled by a Timing Advance (TA) of the UL transmission from different UEs. That is, a UE 521 may start its UL transmission before a nominal time given by the timing of the DL signal that was received by the UE 521.
The timing advance is further illustrated in FIG. 6. In FIG. 6, the UL timing advance is maintained by the network node 510 through timing alignment commands to the UE 521 and the UE 522 based on measurements on UL transmissions from the UE 521 and the UE 522, respectively. Through these timing alignment commands, the UE 521 and the UE 522 are respectively ordered to start their UL transmissions 530, 540 earlier or later, such that the UL transmissions 530, 540 from the UE 521 and the UE 522 are time aligned at the network node 510.
This applies to all UL transmissions except for random access preamble transmissions on Physical Random Access Control Channel (PRACH), i.e. including transmissions on the Physical Uplink Shared CHannel (PUSCH), Physical Uplink Control CHannel (PUCCH), and Sounding Reference Signal (SRS). There is a strict relation between DL transmissions and the corresponding UL transmissions. Examples of this are the timing between a Downlink Shared CHannel (DL-SCH) transmission on PDSCH to the Hybrid ARQ (HARQ) Acknowledgment/Non-Acknowledgment (ACK/NACK) feedback transmitted in UL, either on PUCCH or PUSCH, or the timing between an UL grant transmission on PDCCH to the Uplink Shared CHannel (UL-SCH) transmission on PUSCH.
By increasing the timing advance value for a UE, the UE processing time between the DL transmission and the corresponding UL transmission decreases. For this reason, an upper limit on the maximum timing advance has been defined by 3GPP in order to set a lower limit on the processing time available for a UE. For LTE networks, this value has been set to approximately 667 us. This corresponds to a cell range of around 100 km. Note also that the timing advance value may compensate for the round trip delay.
In LTE Release 10, there is only a single timing advance value per UE and all UL cells are assumed to have the same transmission timing. The reference point for the timing advance is the receive timing of the primary DL cell.
In LTE Release 11, different serving cells used by the same UE may have different timing advance values. In 3GPP, it is currently assumed that serving cells sharing the same timing advance value, e.g. depending on the deployment, will be configured by the network node to belong to a timing advance group, also referred to as a TA group.
It is further assumed that if at least one serving cell of the TA group is time aligned, all serving cells belonging to the same TA group may also use this TA value. Thus, for example, to obtain time alignment for an SCell belonging to a different TA group than the PCell, it is the current 3GPP assumption that initiated random access by the network node may be used to obtain an initial TA-value for this SCell; thus, also for the TA group which the SCell belongs to. The reference point for the TA has not been determined as of yet.
Timing Alignment of Signals Transmitted at the Network Node
There is a requirement on the network node to align the transmit timing of signals transmitted to the same UE by different transmitter ports or branches. This requirement applies to frame timing in transmit (TX) diversity transmissions, MIMO transmission, carrier aggregation and their combinations e.g. antennas involved MIMO transmission, carrier frequencies or cells involved in multi-carriers, CoMP, etc.
In general, for any specific set of transmitter configuration or transmission mode in the network node, the Time Alignment Error (TAE) is defined as the largest timing difference between any two transmitted signals. The purpose of the TAE requirement is to ensure that the UE received signals within a certain period of time. This namely reduces the processing and complexity in the UE. For example, in case of MIMO or TX diversity transmissions, at each carrier frequency, the TAE shall not exceed 65 ns. In another example, in case of intra-band contiguous carrier aggregation, with or without MIMO or TX diversity, the TAE shall not exceed 130 ns. For inter-band carrier aggregation, with or without MIMO or TX diversity, the TAE shall not exceed 1.3 μs. In a further example, in case of CoMP or carrier aggregation when the cells or carriers are physically located in different sites, the TAE may be much larger. Currently no such requirement exists.
SCell Activation and Deactivation
In LTE Release 10, Carrier Aggregation (CA) was introduced, and with this introduction, the concept of SCells. That is additional resources which could be configured or de-configured, and activated or de-activated on a per need basis. The activation or deactivation procedure is described in detail in section 5.13 of 3GPP TS 36.213 Medium Access Control (MAC) protocol specification.
Each SCell is configured with a Cell Index (CE), which may be denoted by SCellIndex. The SCellIndex is an identifier which is unique among all serving cells configured for this UE. The PCell will always have cell index that is 0, and a SCell may have a cell index that is an integer of 1 to 7, i.e. SCellIndex=1, . . . , 7.
One of the areas where MAC cell indexes are used is for activation and deactivation of SCells. In LTE Release 10, the activation or de-activation of MAC cell indexes is defined in section 6.1.3.8 of 3GPP TS 36.213 Medium Access Control (MAC) protocol specification. The activation or de-activation of MAC cell indexes comprises a single octet in turn comprising seven C-fields and one R-field. Each C-field corresponds to a specific SCellIndex. This indicates whether the specific SCell is activated or deactivated. The UE will ignore all C-fields that are associated with cell indexes that are not configured. The activation or de-activation of MAC cell indexes always indicates the activation status of all configured SCells. This means that if the network node wants to activate one SCell, the network node has to include all configured SCells and set them to activated or deactivated even if their status has not changed.
Initial TAC and Subsequent TAC
A timing advance value, i.e. TA value, is used by the UE to offset the UL transmission timing relative a timing reference.
For random access, the UE may assume an initial TA value of zero. The network node measures the time misalignment of the desired UL timing on this cell, and the actual UL timing of the preamble transmission. The network node then creates an initial TA command comprising information which tells the UE how much to advance the UL transmission.
After the random access is successfully completed, the UE will initiate UL transmission on cell i at a time Ti before it receives the DL subframe starting on cell i. The time Ti is deduced from the TA value for the cell i. When receiving these subsequent UL transmissions, the network node also measures the time misalignment of the desired UL timing for this cell and the actual UL timing from the UE on this cell. If measured time misalignment is exceeding a certain value, the network node creates a TA command comprising a delta update, i.e. a timing advance update, for the TA command which is sent to the UE.
In the current release, the initial TA value is an 11 bit long value and is sent in the random access response message. This TA value conveys to the UE how much the UL transmission on a cell should be advanced in relation to a timing reference. In LTE Release 10, this reference timing is the DL of the PCell. Subsequent TA values are updates of the current TA value. The subsequent TA values may be carried in a 6 bit long value and be sent in a MAC control element.
It should be noted that subsequent TA values may be delta updates of the current TA value. Hence, an initial TA value is needed for subsequent TA delta updates to be meaningful. This means that a random access is needed for subsequent TA commands to be meaningful.
In carrier aggregation, the UE may change the cell in a TA group that is used as a timing reference for adjustments of the UL transmit timing. This means that the transmit timing of the new cell to be used as the timing reference may be significantly different compared to that of the previous cell that was used as a timing reference. This change may lead to substantial performance degradation at the network node.
This means that because a change in the timing reference serving cell for a group of serving cells, i.e. TA group, may occur from one subframe to another, there may be an abrupt and significant change in the transmit timing of uplink transmissions on the group of serving cells. This is because the transmit timing of uplink transmissions on the group of serving cells is dependent upon the reference timing of the current timing reference serving cell.
In other words, a large difference between the reference timing of the new timing reference serving cell and the reference timing of the previous timing reference serving cell may cause a sudden and relatively large change in the transmit timing of the uplink transmissions on the group of serving cells. This sudden and relatively large change in the transmit timing may cause problems in the reception of the uplink transmission at the network node, since e.g. the receiver in the network node may not be able to follow such a change from one subframe to another, and consequently lead to performance degradation in the network node.